I made some laboratory tests this morning (April 15, 2000) to get some
indication about the ability to communicate with signals below noise level
using Slow-CW.

I used a calibrated frequency synthesizer (Adret 2230), an 0-120 dB
attenuator in 1 dB steps (Schlumberger BMD500) and my Praecitronic MV61
Selective Level Meter. With a BNC t-connector I fed the normal band noise
including loran lines on 137.500 kHz (+/- 50 Hz) to one side of the t-connector,
and the output of the frequency synthesizer to the other side.

With the attenuator I made sure that a 0 dBm (50 Ohm) signal with the
synthesizer corresponds to a -80 dBu (75 Ohm) signal at the MV62 (plus/minus
1 dB).

The band was quite this morning, with a background noise around -110
dBu (approx. S 4) and Loran lines clearly visible.

Using the 100 Hz bandwidth of the MV62 and the cascaded 250 Hz/500 Hz
CW filters of the IC-746 I checked the signal by ear as well as with the
Spectrogram software with the normal parameters I use for "3-5 second-dot-length"
Slow-CW (5.5k sample rate, 16bit mono, 16384 points FFT = 0.3 Hz resolution,
60 dB scale, 300 ms time scale, 10 x average) and obtained the following
results:

Slow-CW has a 20 dB signal level advantage over normal (aural CW), which
means that the minimum detectable and/or readable Slow-CW signal that might
just allow communication lies 20 dB below the signal, that can just be
detected and/or decoded by a trained CW-operator's ear (yes, I consider
myself to be a trained CW operator ...). If I consider the "CW-operator's
ear/brain bandwidth" to be 30 Hz, this roughly corresponds to the bandwidths
used (0.3 vs 30 Hz).

I would be interested to get your comments or own measurements on this
subject. I do not yet have sufficient experience with Spectran to make
full advantage of this software, so I would like to hear about that software
as well.